Radiative Torques Alignment in the Presence of Pinwheel Torques

We study the alignment of grains by radiative torques and pinwheel torques taking into account internal relaxations and thermal flipping. We identify with radiative torques the torques arising from anisotropic radiation, while all torques that are fixed in grain body axes, including the radiative torques arising from extinction of isotropic radiative field, we identify with pinwheel torques. We discuss new type of pinwheel torques, namely, regular torques arising from grain emission, as well as, pinwheel torques arising from the interaction of grains electrons in hot plasma. We show that both types of torques are long-lived, i.e. may exist longer than the grain damping time. We also show that the torques, first introduced by E. Purcell, arising from photoelectric emission of electrons, can also be long-lived. The other two types of Purcell’s torques, namely, those arising from H2 formation and variations of the grain accommodation coefficient, we identify with short-lived torques, although their actual life-time is rather uncertain. With the uncertainties in the amplitude of the torques and the variety of the astrophysical conditions involved, we consider separately the effect of the long-lived and short-lived pinwheel on the alignment induced by radiative torques. For long-lived pinwheel torques, we observe that new attractor points appear. Grains at these points rotate fast mostly due to pinwheel torques and are perfectly aligned with their long axes perpendicular to magnetic field. The percentage of grains in such a state depends on the life-time of pinwheel torques. For short-lived pinwheel torques, we find a new effect of the suppression of grain flipping by strong radiative field. Such a field, characteristic of circumstellar regions, decrease the rate of grain flipping at a low angular momentum attractor point, increasing the alignment. The mechanisms of alignment we discuss here do not involve paramagnetic relaxation. The only effect of magnetic field on grains that we consider is Larmor precession. We discuss implications of the alignment for interstellar medium and circumstellar regions. Subject headings: Polarization -dust extinction -ISM: magnetic fields